1. Field of the Invention
This invention relates to a discharge gap device in which discharge occurs between conductors, and more specifically to a discharge gap device which is provided between an antenna input terminal or a secondary side earth and a commercial power source, or between the power lines of a commercial power source as a ground discharge countermeasure for electrical equipments such as a television set, a video cassette recorder, and a television and video compound device, and to its mounting structure.
2. Description of the Related Art
A CR (capacitor and resistor) compound part 21 whose circuit arrangement is as shown in FIG. 7 is known as a discharge gap device (for instance, B2R131C131, R1-2M121MF, etc. manufactured by Murata Manufacturing Co., Ltd.).
The CR compound part 21 comprises a discharge gap unit 22, a resistor 23, and a capacitor 24 which are connected in parallel to one another.
FIGS. 8(a) and 8(b) are external views of the CR compound part 21 shown in FIG. 7.
As shown in FIGS. 8(a) and 8(b), in the CR compound part 21, lead wires 27 and 28, which are to be inserted into a printed circuit board (not shown) , are welded on a surface 29a of a dielectric 29 with soldering patterns 25 and 26 which are conductors, and a resistor 23 is connected between the soldering patterns 25 and 26. On a rear surface 29b of the dielectric 29, a silver (conductor) pattern 30 is printed or bonded, and the entire rear surface is covered with resin (not shown) so as to cover the silver pattern 30.
With the discharge gap device thus designed, ends 25a and 26a of the soldering patterns 25 and 26 form the discharge gap unit 22, while the dielectric 29 is held between the soldering patterns 25 and 26 and the silver pattern 30 to form the capacitor 24. Thus, as shown in FIG. 7, a discharge circuit has been formed in which the discharge gap unit 22, the resistor 23, and the capacitor 24 are connected in parallel to one another.
FIG. 9 is a circuit diagram showing the discharge gap device (CR compound part 21) applied to a television set.
As shown in FIG. 9, the current of a commercial power source 16 is applied to a rectifier circuit 17, and the current rectified by the circuit 17 is supplied to a power source circuit (or switching power source circuit) 18. The output current of the circuit 18 is supplied to a variety of load circuits.
A tuner 19 of the television set is connected through the discharge gap device 21 to the aforementioned commercial power source 16. Normally, the discharge gap unit 22 is in non-conduction state; that is, the tuner 19 is insulated from the commercial power source 16. A filter 20 is provided between the CR compound part 21 and the tuner 19. The filter 20 is made up of a coil and the like to cut off high frequency components.
In FIG. 9, the one-dot chain line A indicates a primary power source side between the commercial power source 16 and the switch power source circuit 18, and the other one-dot chain line B indicates a secondary power source side at the rear stage of the switching power source circuit 18.
An earth (ground) E1 forming the rectifier circuit 17, and an earth E2 which is connected through a capacitor C between the rectifier circuit 17 and the switching power source circuit 18 mean the earth of the primary power source side A, and are at the same potential. Furthermore, an earth E3 forming the switching power source circuit 18, and earths E4 and E5 of the tuner 19 mean the earth of the secondary power source side B. and are at the same potential.
With the above-described circuit, when ground discharge occurs, the load circuits are prevented from damage as follows:
For instance, when the occurrence of ground discharge causes and high voltage is applied through the antenna (not shown) to the antenna input terminal 19a of the tuner 19, the gap of the discharge gap unit 22 of the CR compound part 1 connected to the tuner 19 is made conductive by the discharge, so that the high voltage is applied to the commercial power source 16. The high voltage does not go to the side of the secondary power source B, whereby the load circuits are prevented from damage.
FIG. 10 is a circuit diagram showing the case where the two above-described conventional CR compound parts 21 are used.
In this discharge gap device, the two CR compound parts 21 and 21 are connected in series to each other, and one of the CR compound parts 21 is shunted by a capacitor C.
The two compound parts 21 and 21 are used mainly, for instance for a television set combined with a video cassette recorder, to meet the safety standard.
For instance, the UL (Underwriters laboratories) standard in U.S.A. is as follows: In the safety standard for a popular television set, it is necessary that the gap of the discharge gap unit 22 is at least 1.6 mm; and in the safety standard for a television set combined with a video cassette recorder, it is at least 3.2 mm because double insulation is required.
In the discharge gap device of the CR compound part 21, the gap G (between the ends 25a and 26a of the soldering patterns 25 and 26) of the discharge gap unit 22 is set to 2.6 mm. Therefore, in order to provide 3.2 mm, it is necessary to additionally manufacture a CR compound part. Hence, in order to meet the UL standard, two 1.6 mm-gap CR compound parts 21 are used to make the gap 3.2 mm.
Incidentally, in the above-described related art, the CR compound part 21 is employed as the discharge gap device. Hence, in the case where it is required to use a variety of resistances, it is necessary to newly manufacture CR compound parts.
Furthermore, the CR compound part 21 includes the capacitor 24, and in the case of the ordinary connection, high frequency components pass through the CR compound part through the capacitor 24, as a result of which high frequency components from the commercial power source is applied to the secondary circuit of the tuner 19, or high frequency components from the secondary circuit of the tuner 19 are superposed on the power.
Hence, sometimes, it is necessary to connect the CR compound part 21 and a filter 20 to cut off the high frequency components.
Furthermore, in order to meet the safety standard of a television set combined with a video cassette recorder, the two CR compound parts 21 are employed; that is, the number of components is increased as much.